1. Field of the Invention
The present invention relates to a substrate processing method and apparatus, and more particularly to a substrate processing method and apparatus useful for a pre-plating treatment which may be performed before electroless plating for the formation of an interconnects-protective layer on exposed surfaces of embedded interconnects of a conductive material, such as copper, silver or gold, embedded in fine interconnect trenches formed in a surface of a substrate, such as a semiconductor wafer.
The present invention also relates to a substrate processing unit useful for performing processing, such as plating and a pre-plating treatment, of a front surface (lower surface) of a substrate while holding the substrate with the front surface facing downward and with a peripheral portion of the front surface sealed. In particular, a substrate processing unit is used as a pre-plating treatment unit to perform a pre-plating treatment in advance of electroless plating for a formation of an interconnects-protective layer on exposed surfaces of embedded interconnects of a conductive material, such as copper, silver or gold, embedded in fine interconnect trenches formed in a surface of a substrate, such as a semiconductor wafer.
2. Description of the Related Art
As a process for forming interconnects in a semiconductor device, a so-called “damascene process”, which comprises embedding a metal (electric conductor) into trenches for interconnects and contact holes, is coming into practical use. According to this process, aluminum, or more recently a metal such as silver or copper, is embedded into trenches for interconnects and contact holes previously formed in an interlevel dielectric of a semiconductor substrate. Thereafter, extra metal is removed by performing chemical mechanical polishing (CMP) so as to flatten a surface of the substrate.
In a case of interconnects formed by such a process, for example copper interconnects formed by using copper as an interconnect material, embedded copper interconnects have exposed surfaces after the flattening processing. In order to prevent thermal diffusion of such interconnects (copper), or to prevent oxidation of such interconnects (copper) e.g. during forming thereon an insulating film (oxide film) under an oxidizing atmosphere to produce a semiconductor device having a multi-layer interconnect structure, it is now under study to selectively cover the exposed surfaces of interconnects with an interconnects-protective layer (cap material) composed of a Co alloy, a Ni alloy or the like so as to prevent thermal diffusion and oxidation of the interconnects. Such an interconnects-protective layer of a Co alloy, a Ni alloy or the like can be produced e.g. by performing electroless plating.
As shown in FIG. 19, for example, fine recesses 4 are formed in an insulating film 2 of SiO2 or the like which has been deposited on a surface of a substrate W such as a semiconductor wafer. A barrier layer 6 of TaN or the like is formed on the entire surface, and then copper plating, for example, is carried out onto the surface of the substrate W to fill the fine recesses 4 with copper and deposit copper on the insulating film 2. Thereafter, CMP (chemical mechanical polishing) is carried out onto the surface of the substrate W so as to flatten the surface of the substrate, thereby forming interconnects 8 composed of a copper film in the insulating film 2. Thereafter, an interconnects-protective layer (cap material) 9 composed of a Co—W—B alloy film is formed e.g. by performing electroless plating selectively on the surfaces of interconnects (copper film) 8 to protect interconnects 8.
A common electroless plating method for the selective formation of the interconnects-protective layer (cap material) 9 of Co—W—B alloy film on the surfaces of interconnects 8 generally involves the following process steps: First, the substrate W such as a semiconductor wafer, which has undergone the CMP treatment, is immersed in an acid solution e.g. of 0.5M H2SO4 at the solution temperature of e.g. 25° C. for e.g. one minute to remove CMP residues, such as copper, remaining on a surface of an insulating film 2. After cleaning the surface of the substrate W with a cleaning liquid such as ultrapure water, the substrate W is immersed in a mixed solution, e.g. of 0.005 g/L PdCl2 and 0.2 ml/L HCl, at the solution temperature of e.g. 25° C. for e.g. one minute to adhere Pd as a catalyst to the surfaces of interconnects 8, thereby activating the exposed surfaces of interconnects 8. Next, after cleaning the surface of the substrate W with a cleaning liquid such as ultrapure water, the substrate W is immersed in a solution containing e.g. 20 g/L of Na3C6H5O7.2H2O (sodium citrate) at the solution temperature of e.g. 25° C., thereby carrying out neutralization treatment of the surfaces of interconnects 8. Thereafter, after washing the surface of the substrate W with ultrapure water, the substrate W is immersed in a Co—W—B plating solution at the solution temperature of e.g. 80° C. for e.g. 120 seconds, thereby carrying out selective electroless plating (electroless Co—W—B cap plating) onto the activated surfaces of interconnects 8. Thereafter, the surface of the substrate W is cleaned with a cleaning liquid such as ultrapure water. The interconnects-protective layer 9 composed of a Co—W—B alloy film is thus formed selectively on the surfaces of interconnects 8 to protect interconnects 8.
As described above, when forming an interconnects-protective layer (cap material) composed of a Co—W—B alloy film by electroless plating, a catalyst-imparting treatment for imparting a catalyst, for example Pd, to the surfaces of interconnects is carried out in advance. Further, removal of CMP residues, e.g. copper, remaining on an insulating film, which treatment is necessary for preventing an interconnects-protective layer from being formed on the insulating film, is carried out by usually using an inorganic acid, such as H2SO4 or HCl. Accordingly, it is necessary to carry out a neutralization step immediately before performing plating to stabilize the plating process.
In order to securely perform uniform plating in the necessary area of the surface of a substrate after performing a pre-plating treatment, it is necessary to securely impart a catalyst only to that area (plating area) in the catalyst-imparting treatment, and effect a neutralization treatment, etc. over the whole area to which a catalyst has been imparted.
In conventional plating apparatuses, however, a pre-cleaning treatment (chemical cleaning), which is carried out prior to a catalyst-imparting treatment, a catalyst-imparting treatment and a cleaning treatment (neutralization treatment) after the catalyst-imparting treatment, are generally carried out by using devices each having the same construction. Accordingly, the respective areas of a substrate to be subjected to the pre-cleaning (chemical cleaning), to the catalyst-imparting treatment and to the cleaning (neutralization) after the catalyst-imparting treatment are basically the same. With such a conventional apparatus, due to a device error, a variation in positioning of a substrate when it is held, etc., there is a case where that area of the substrate to which a catalyst will be imparted is not entirely pre-cleaned (with a chemical) or a case where the area of the substrate to which the catalyst has been imparted is not entirely cleaned (neutralized) later, whereby plating cannot be effected securely in the necessary area of the surface of the substrate.
The above-described pre-plating treatments are usually carried out by holding a substrate with its front surface facing downward (face down) while sealing a peripheral portion of the front surface with a seal ring, and allowing the front surface (lower surface) of the substrate to be in contact with a pre-plating treatment liquid. In a conventional substrate processing unit, in particular a pre-plating treatment unit for carrying out such a pretreatment, there is no space between a substrate and a seal ring, i.e. on the front surface (lower surface) side of the substrate, for taking in and out a robot hand when holding the substrate while sealing the peripheral portion with the seal ring or when carrying the substrate out after the treatment. It is therefore a usual practice with such a unit to attract and hold on the back surface (upper surface) side of a substrate by a vacuum hand or gripper hand when transferring the substrate and placing the substrate at a predetermined position on a seal ring or taking the substrate away from the seal ring.
However, holding and transferring a substrate by vacuum attraction with a vacuum hand, foe example, generally involves a considerable loss of time, taking much time to hold the substrate. Further, there is always a risk of fall of the substrate when the substrate held by a vacuum hand is transferred at a high speed. It is therefore necessary to use a low substrate transfer speed in order to avoid the risk of fall of substrate, leading to a lowered throughput.
It may be considered to raise the substrate transfer speed by separately taking safety measures against the fall of substrate, for example, provision of a mechanical chuck. Such safety measures, however, would make the apparatus complicated. In addition to the foregoing, holding a substrate by vacuum attraction of the central portion of even the back surface could cause generation of particles. There is therefore a demand for a technology that makes it possible to hold a substrate without contact with the other portion of the substrate other than a particular peripheral portion, a so-called edge-cut portion.